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Carl Sagan's 5 Greatest Contributions to Science

"Carl Sagan, more than any contemporary scientist I can think of, knew what it takes to stir passion within the public when it comes to the wonder and importance of science," Bruce Alberts, president of the National Academy of Sciences, said on the day of Sagan's death.

As the creator of the television series Cosmos: A Personal Voyage, a reboot of which is in the works for early 2014 on FOX, Sagan is widely credited with making science popular. You can bet there would be no Bill Nye or The Big Bang Theory without him.

He was also a Pulitzer Prize-winning author, and contributed to some of NASA's most prominent missions in the 1970s and '80s. Sagan died on Dec. 20, 1996 of pneumonia, a complication of the bone marrow cancer myelodysplasia.

Of course, Sagan was not without his faults. As David A. Hollinger wrote in The New York Times, "Sagan [was] a dreadful narcissist and an irresponsible parent until middle age, when his third wife, Ann Druyan, apparently transformed him into something of a mensch." And then there's the "pseudo-Egyptian temple" where he lived, complete with an abundance of surveillance cameras and an automated iron gate.

Nonetheless, Sagan changed the face of modern science, from his groundbreaking research to his bestselling works of nonfiction. Here, we unpack five of Sagan's greatest contributions to science.

For a more in-depth look at Sagan's life, take a peek at this tribute to the man from the Times.

1. Cosmos: A Personal Voyage

Sagan is best known for the 13-episode, documentary-style television show Cosmos: A Personal Voyage, which went on to win a Peabody Award. Broadcast in 1980, the series examined complex scientific concepts in a way that the average American could understand, with topics ranging from the history of astronomical observation to the structure of the atom.

Sagan's legendary sense of wonder permeated the show and made these concepts appealing to a wide audience, cementing his fame as the public face of planetary science. His status as a "celebrity-scientist" extended well beyond Cosmos' run. He also appeared on The Tonight Show with Johnny Carson a whopping 26 times, calling the late-night talk show "the biggest classroom in history."

In July, astrophysicist Neil deGrasse Tyson announced that he would be rebooting the series as Cosmos: A Spacetime Odyssey. It is scheduled for a Spring 2014 premiere on FOX.

2. Space Exploration

As a leading consultant to NASA, Sagan participated in groundbreaking missions that advanced our understanding of the solar system.

He was the first to suggest that the planet Venus did not possess a balmy tropical climate similar to Earth's, as scientists had previously believed. By studying radio emissions from Venus, he theorized that the planet might in fact have a surface temperature of 900 degrees Fahrenheit and a crushing atmosphere. He would later help design and manage NASA's Mariner expeditions to Venus, which confirmed this theory.

Along with his former student James Pollack, Sagan also shifted scientific perceptions of the planet Mars. Some scientists had previously suggested that color variations on Mars' surface could be indicative of plant life growing seasonally on the planet's face; Sagan and Pollack hypothesized that these changes were actually the result of Martian dust caused by wind storms. The Mariner 9 and Viking expeditions to Mars in the 1970s confirmed this theory.

Sagan was a member of the Mariner 9 imaging team, and helped select the landing sites for Viking 1 and Viking 2. These spacecraft were significant for being the first to orbit and subsequently land on Mars successfully.

Sagan was also a member of the science teams selected by NASA for Pioneer 10 and 11, which flew by Jupiter and Saturn, and for Voyager 1 and 2, which visited Jupiter, Saturn, Uranus and Neptune, and took the first photographs of the solar system from beyond Pluto. In September 2013, Voyager reached another milestone when it became the first spacecraft to reach interstellar space — on less memory than an iPhone 5.

In 1973, Sagan was quoted as saying, "Even today, there are moments when what I do seems to me like an improbable, if unusually pleasant, dream."

3. Belief in Extraterrestrial Life

Although Sagan believed in the existence of extraterrestrial life, he took a firm stand against "pseudosciences" such as astrology and alternative medicine, and against the cult of UFO sightings and alien abductions. In 1969, he held an American Association for the Advancement of Science symposium intended to devalue the notion that UFO sightings were proof of visiting spacecraft.

Sagan first achieved national attention as a proponent of extraterrestrial life in 1966, when he published Intelligent Life in the Universe with Russian astronomer I. S. Shklovskii. Soon after, he wrote a National Geographic article on the potential for life on other planets.

As a member of NASA's Voyager team, he suggested putting a message aboard the spacecraft on the chance that extraterrestrial beings would eventually come across it. He attached a 12-inch phonograph record, sheathed in a protective aluminium jacket, to the outside of the craft. According to The New York Times, it included "greetings from people in many languages and from whales, a 12-minute sound essay, 90 minutes of music and a series of blips to be decoded into black-and-white and color photographs."

Sagan was also involved with the Search for Extra-Terrestrial Life Institute (SETI), which is now home to the Carl Sagan Center for the Study of Life in the Universe.

4. Political Advocacy

Sagan strongly believed that human beings were altering their environment in a way that would become unsustainable, and was an early believer in the concept of global warming.

He was also an early opponent of Ronald Reagan's Cold War-era Space Defense Initiative, also known as "Star Wars." In 1982, Sagan, along with former students James Pollack and Brian Toon, as well as colleagues Rich Turco and Tom Ackerman, concluded that "the smoke from as few as 100 burning cities, when lofted into the stratosphere, could lead to severe global cooling."

In the journal Science they dubbed this cooling effect "nuclear winter" and suggested that "even a less-than-full-scale nuclear exchange… could cause global cooling and collapse of agriculture" that could potentially endanger the entire planet. This theory, Sagan argued, rendered the concept of nuclear war obsolete. Sagan later led a delegation to meet with Pope John Paul II, who then issued a papal statement against building nuclear arsenals.

Sagan's nuclear winter theory was widely debated, and was not without its problems. In 1990, when Iraq threatened to set fire to the oil wells of Kuwait, Sagan was convinced that the petrochemical smoke from these fires would generate a small-scale nuclear winter. Although few of his colleagues supported the hypothesis, Sagan went public with this prediction. In January 1991, Iraq set fire to Kuwait's oil fields, which disrupted the coastal ecosystem but had no effect on the climate, even on a local scale.

5. Published Work

Like Cosmos: A Personal Voyage, Sagan's published work brought scientific concepts to a wider audience. He penned roughly 20 books as an author or co-author, ranging in scope from extraterrestrial life (Intelligent Life in the Universe) and the evolution of human intelligence (The Dragons of Eden) to the history of space exploration (Pale Blue Dot) and "the assorted flotsam and jetsam of pseudoscience" (The Demon-Haunted World). The Dragons of Eden: Speculations on the Evolution of Human Intelligence earned Sagan a Pulitzer Prize in 1978.

In the 1980s, Sagan received an unprecedented $2 million advance by Simon & Schuster to write a science fiction novel. The bestselling Contact was published in 1984, and became a 1997 film starring Jodie Foster.

Sagan's nonfiction accompaniment to Cosmos stayed on the bestseller list for more than a year. In The New York Times Book Review, James Michener describedCosmos as ''a cleverly written, imaginatively illustrated summary of his geological, anthropological, biological, historical and astronomical ruminations about our universe,'' and added, ''His style is iridescent, with lights flashing upon unexpected juxtapositions of thought.''

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